It is also known that spatial coherence is a factor in laser light sources, e.g., excimer and other gas discharge laser light sources, e.g., molecular fluorine laser light sources properly performing according to specification in, e.g., the use of such light, e.g., in the DUV range, for, e.g., exposure of photoresists on integrated circuit wafers. The makers of such integrated circuit lithography tools are demanding tighter and tighter specifications for spatial coherence. Some lasers manufactured by applicants' assignee, e.g., XLA lasers, are borderline passing tests for spatial coherence according to current practice and may become even more susceptible to being out of specification in the future as specifications are defined more tightly. In the past the metrology used to define spectral coherence was to, e.g., find a point with maximum interference fringe contrast in the laser beam and use that as a measure of spatial coherence. However this data point does not represent the property of coherence of the whole beam. Applicants have, therefore, determined that a better means of measuring beam spatial coherence is needed and propose such a method in the present application.